Neutralized radio-frequency amplifier



July 16, 1946. E, HI PS 2,404,18

NEUTRALIZED RADIO FREQUENCY AMPLIFIER Filed Jan. 11, 1943 Patentecl July 16, 1946 NEUTRALIZED RADIO-FREQUENCY AMPLIFIER Willis Phillips, Deerfield, Ill., assignor to Zenith Radio Corporation, Chicago, 111., a corporation of Illinois Application January 11, 1943, Serial No. 471,960

13 Claims. 1

This invention relates to neutralized radio frequency amplifiers and particularly to such amplifiers which are employed at ultra-high frequencies, of the order of above thirty megacycles.

It is well known that the electrodes of a thermionic tube, and particularly the plate and control grid, have a definite capacity through which the output and input circuits are coupled together. To prevent excessive feed-back and regeneration, a neutralizing condenser having approximately the same capacity as that existing between the grid and plate is arranged to supply an equal amount of energy of opposite phase from the plate to the control grid. In the case of push-pull amplifiers, the plate of each tube is usually connected through a neutralizing condenser to the control grid of the other tube.

When such conventional circuits are embodied in ultra-high frequency amplifiers, considerable difiiculties are experienced. This is particularly true where large tubes are employed because the neutralizing leads or conductors have, in that case, and particularly in the case of push-pull amplifiers, to be of a length which is a substantial fraction of the wave length. At ultra-high frequencies the neutralizing conductor has a very substantial reactance and the neutralizing condenser and the neutralizing conductor have a very large distributive capacity to ground, a ca-. pacity which may approach that of the neutralizing condenser itself.

I find that in some cases the neutralizing conductor and the plate of the neutralizing condenser to which it is attached acquire very high voltages. In some cases, the condenser arcs over, and arcs extend out of the conductor and into the air. This voltage may become so large in high-power ultra-high frequency amplifiers as to be the limiting factor in determining the maximum power that may be produced by such an amplifier. Furthermore, the input tuned circuit is effectively shunted by the inductance of the neutralizing conductor and the distributive capacity of the conductor and the neutralizing condenser so that the inductance of the input circuit must be seriously limited. In many cases, it is found that even though the capacity of the input circuit be eliminated it is not possible to tune the input circuit to the ultra-high frequency to which it is desired to operate the amplifier.

The principal object of the invention is to provide a neutralized ultra-high frequency amplifier in which these disadvantages are obviated.

A furthe obiect of the invention is to provide a neutralized ultra-high frequency amplifier in which the neutralizing conductor and the neutralizing condenser have no excessive voltage.

Other objects, advantages and capabilities of the invention will appear from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a wiring diagram of a single tube ultrahigh frequency amplifier provided with a neutralizing circuit embodying my invention;

Fig. 2 is a similar diagram of a modified form of the invention;

Fig. 3 is a wiring diagram of an ultra-high frequency push-pull amplifier provided with neutralizing circuits embodying my invention;

Fig. 4 is a diagrammatical view of a physical embodiment of a high-power, ultra-high frequency amplifier constructed in accordance with the wiring diagram of Fig. 3, and

Fig. 5 shows an alternative specific connection in the arrangement shown in Figure 3.

Referring to the drawing and particularly to Fig. 1 thereof, the reference numerals Ill and H designate the inductance and tuning condenser of the input circuit which is connected to the control grid !2 of the thermionic tube [3. The plate I4 of the tube 13 is connected to one terminal of the tuned circuit l5 which has its other terminal connected by a by-pass condense IE to the cathode H. A source of high voltage I8 is connected to the junction point of circuit l5 and condenser H5. The .plate [4 is connected through a neutralizing condenser I9 to a conductor 2t which is connected to the low side of the inductance it. The conductor 26 has at ultrahigh frequencies a relatively high reactance which is illustrated by the coil 2|. The reactance may attain very high values and consequently it is shown as a full line coil 2! notwithstanding-the fact that the conductor 29 is a solid piece of copper.

In Fig. 1 I illustrate as the capacity 22 the distributive capacity of the conductor 20 and the plate of the condenser l 9 to which it is attached. This distributive capacity provides a low reactance path at ultra-high frequencies. The value of capacity 22 may approach the value of the neutralizing condenser is and consequently it is shown in full lines as a condenser.

In accordance with the present invention I connect an inductance 23 between the conductor 2!] and the plate of the condenser l9 to which it is attached and ground. This inductance 23 may be of the order of a few microhenries.

I have found that the objectionable high volt- -which .the conductor '20 is attached.

:proper neutralization.

aeoaiss age produced in the conductor .20 and in the plate of the condenser id to which it is attached results from the fact that the inductive reactance 2|, together with the capacitive reactance Z2, constitutes, at ultra-high frequencies, a series resonant circuit. My introduced inductance 23 destroys this serie resonant circuit. Preferably it constitutes, with thedistributive capacity 22, aparallel resonant circuit whichhas a high resistance to the ultra-high frequency current. That is, the inductance or inductor 23 is connected in parallel with the distributive capacity 22 of said conductor 20 and condenser l9 and one terminal of inductance 23 is connected adjacent the .juncticn .of conductor 25) and condenser l9. 'Beforetheintroduction of my inductance 23, a large resonant current flowed from the conductor 2Q :to g o through the easy path provided by the distributive capacity 22, and this high current resulted in the excessively high voltage of the conductor Zfiandof the plate of 'the condenser ill to which the conductor dais-connected. The parallel resonantcir'cuit 22,23 cuts downthis ultra-high frequenoy'current to aluminium, and thus the objectionable high voltage is destroyed.

The'inductance value of the inductor23is best determined practically. Thus I introduce'an inductor'23 which is obviously too large and I ad- 'ju'st thecondenser l 9 for optimum neutralization.

I determinethevoltage of the conductor 20 and the 'plate "'of'the condenser 59 to which it is attached. In high-power installations I may contactth'e plate of the condenser l 'with ametal rod held on an insulating "handle. Onwithdrawing therod'an arc'will extend between the condenser l9 and the end of the rod, owing to becomeslessand less, and the final steps of adjustment maybe-performed with the aid of a glow lamp'untiL when I have an inductor 2-3 of the correctinductance value,'no excessive voltage can 'be'discovered on the-plate of the condenser I 9' to When that end is attained, the distributive capacity 22 and the inductor 23 are believed to constitute a parallel resonant-circuit at the ultra-high fre- 'quency. Asthe value of inductance 23' is progressivelydecrease'd; the capacity of the condenser l9 has to :be "progressively increased to provide When set correctly, the condenser l9 has a capacityapproximately-equal to that "existing between the grid iz and the plate l4.

In Fig: 2 I have shown the wiring-diagram embodying "my'invention in another conventional neutralizing circuit. In this'circuit the neutralizingcondenser 19 has one of its terminals connected tothe 'lowend'of the'output tuned'circuit l and its other terminal connected bymeans'of a'conductor '25] to the control grid 92 through the unnumbered condenser. In this case the inductive'reactance of the conductor 20 is illustrated as the coil 2! and the distributive capacity of the "conductor 29 and the plate of the condenser I'EJ towhich it is attached is also illustrated as the capacity22.

My introduced-inductor 23 has one of itsterminalsconnected to the plate of the condenser I9 to-which the conductor 2!] is connected and its other-terminal connected to ground. At the final adjustment'the inductor 23 constitutes With the capacity'ZZ aparallel circuit resonant at the ultra-high frequencyand the capacity of the condenser it closely approximates the capacity between the plate l4 and the control grid I2.

In Fig. 3 I have shown a preferred manner in which m invention is embodied in a push-pull amplifier provided with neutralizing circuits extending from the plate of one tube M to the control grid of the other tube i2. Each neutralizing circuit comprises a neutralizing condenser [9 connected to the plate of one tube, which neutralizin condenser is connected by a conductor Zii to the grid of the other tube. Here again the inductive reactance of the conductor 20 is illustrated as a coil 2i and the distributive capacity of each conductor .23 and the plate of the condenser; E9 to which .it is attached is illustrated as aicapacityzl .This.circuit is preferred for high-power, ultrahigh-frequency amplifiers since the neutralizing circuits can be kept relativel short. Nevertheless, since the tubes used must be physically large for high-power amplifiers, the conductors 29 havesubstantial length an'd'objectionable high voltages are found onthesecon'ductors and on the plates'of'the condensers to'which'they are connected. "It will-be'understood that the objectionable high voltage increases with increase offrequency.

In this case, in Fig. 3also,'Imay correctthe circuit by connecting a'coil 23 between each .condenser '99 and ground .in the manner described above in relation to Figs'l and 2. In practice, however, I prefertto connect arsingle :inductor23' between .thetwoxcondensers L9, in "the f manner shown in. Fig. '3. The center of theindu'ctor' 23' isa neutral'orrzero point and it'may'be grounded if desired.asshown..iniliigurefi. It is thus seen iromzhiguresit and '5 that the inductorfill may constituteeithena single inductor'or in effect two inductors :for achieving "the purposes of the present; invention. The steps of determining the proper.inductancevalue 'of the inductor 2.1% are the same 'asxpreviously described. In the *final condition of: adjustment each condenser l9 is'adjusted to correspond to the plate to grid .capacity ofthe tubesandeach half of the inductor .23 constitutes, with :the adjacent capacity :22, a parallel resonant circuit at the .ultra high .q CY- InFig, 4.: show diagrammatically a physical embodiment of a high-power, ultra-high fre quenc amplifier constructed; in. accordance' with the wiring diagram of Fig. 3. This amplifier comprises two;highpower, water-cooled triodes it which include two depending copper cylinders Z a-which; are bridged across by-a metal bar 25 to which .thehigh'voltagesource la is connected by means of acon'ductor -2i'igthe cylinders 24 being connected to the plates of the tubes. Theoutput tuned circuitt5. is constituted by adjustable condenserplates-fil', a metal bar 23 slidably mounted onthe cylindersfikand a swingable loop or stirrup 29 -connected.tothe output leads -30. The neutralizingcondensers ware constituted by arcuate plates .135 which are aadjustably. mounted so that they can be broughtinto proper spaced relation with cylinders :26. The arcuate plates 3| are connected by'the conductors-20 togridterminals of .the tubes 13, the conductorsrbeing arranged CZiSS'rClTOSS fashion andin spaced relation. The inductor is constituted bytwometa1 bars:32, the ends of whichiare connectedto the-grid .connectors of the tubes. The 'bars 32 are bridged, remote from these ends, by a metal block 33Jarrangedto establish desired induct-ance for the inductor Iii. The capacitorll is'constitutedsby an adjustable grounded plate 34 arranged to move between the bars 32 to provide proper tuning capacity for the grid circuit. Grid energy is supplied to the inductor ID by means of a loop 35 arranged immediately below the bars 32. Filament energy is provided by the coaxial lines 36, the interior conductor being connected t one filament terminal on the tube I3 and the outer conductor being connected to the other filamem terminal of the tube l3.

The inductor 23' is connected between the two plates 3|. This inductor may take the form of a stout copper wire formed with a few coil turns depending upon the necessities of the installation. In practical cases it may have an inductance of a few microhenries, which is sufiicient at ultrahigh frequencies to provide a high reactance. Thus, at 50 megacycles one millihenry has a reactance of more than three hundred thousand ohms, and ordinarily a mere fraction of that inductance is sufficient to provide the desired effect.

It will be noted that in the single-tube amplifler shown in Figs. 1 and 2 the input tube circuit I0, H is shunted by the tuned circuit 2!, 22 in the absence of the added inductor 23. In some cases, even with a very small inductance l0 and the omission of a physical condenser H, it is impossible to get the frequency sufiiciently high. The provision of the added inductor 23 or 23 as the case may be greatly reduces the shunt loading across the tuned circuit and permits a more favorable ratio of L to C and allows the amplifier to be operated at a higher frequency. Furthermore, the loading of the plate tuned circuit 15 by the shunting effect of capacities is and 22 is reduced by the provision of the inductor 23 permitting higher frequency operation and reducing losses in the tank circuit H5. The same effects are attained in the push-pull circuit shown in Fig. 3 and illustrated in Fig. 4.

Although the invention has been described in connection with details of specific embodiment thereof, it must be understood that such details are not intended to be limitative of the invention except insofar as set forth in the accompanying claims.

I claim:

1. In an ultra-high frequency power amplifier including a thermionic tube and a neutralizing condenser and a neutralizing conductor for supplying plate energy to a control grid, the combination with said neutralizing elements of an inductor of substantially no resistance arranged in parallel with the distributive capacity of said conductor and condenser and connected thereto adjacent their junction.

2. In a push-pull ultra-high frequency power amplifier including a pair of thermionic tubes, a pair of neutralizing condensers and neutralizing conductors connecting the plate of each tube to the control grid of the other tube, the combination with said neutralizing elements of an inductor arranged in parallel with the distributive capacity of each conductor and condenser and connected adjacent the junction of each conductor and its condenser.

3. In a push-pull ultra-high frequency power amplifier including a pair of thermionic tubes, a pair of neutralizing condensers and neutralizing conductors connecting the plate of each tube to the control grid of the other tube, the combination with said neutralizing elements of an inductor connecting a point adjacent the junction between one said condenser and its associated conductor to a point adjacent the junction be- 6 tween the other said condenser and its associated conductor.

4. In an ultra-high frequency power amplifier including a neutralizing condenser and a neutralizing conductor which develops excessively high voltages as a result of the reactance of said conductor at ultra-high frequencies, the combination of an inductor of substantially no resistance in parallel with said conductor and connected to a point of excessive high voltage, said conductor having sufficient reactance to diminish substantially said excessively high voltage.

5. A neutralizing circuit including a neutral-. izing condenser and a. neutralizing conductor, said condenser and conductor having a substantial distributive capacity and said conductor having sufficient length whereby at ultra-high frequencies said distributive capacity and the impedance of said conductor tend to constitute a resonant circuit with the production of excessively high voltages on said condenser and conductor, in combination with an inductor of sufficient reactance connected adjacent the junction between said condenser and conductor to provide with said distributive capacity a parallel resonant circuit at said frequencies whereby the first said resonant circuit is effectively destroyed and the excessively high voltages resulting therefrom are prevented.

6. A neutralizing circuit including a neutralizing condenser and a neutralizing conductor having a substantial distributive capacity and said conductor having sufficient length whereby at ultra-high frequencies said circuit resonates, producing excessively high voltages therein, and a reactor of substantially no resistance connected to said circuit adjacent the junction between said condenser and conductor for effectively destroying the resonance thereof and effectively nullifying said excessively high voltages.

'7. In a push-pull, ultra-high frequency power amplifier including a pair of thermionic tubes, a pair of neutralizing condensers and neutralizing conductors connecting the plate of each tube to the control grid of the other tube, each condenser and conductor having a high distributive capacity and each conductor having sufficient length whereby each conductor and its condenser tend to resonate at ultra-high frequencies, producing excessively high voltages therein, and a reactor of substantially no resistance connecting points adjacent said condensers whereby the tendency to resonate and to produce ultra-high frequencies, is effectively nullified.

8. In an ultra-high frequency power amplifier circuit, including at least one thermionic tube having plate and grid and a neutralizing condenser and a neutralizing conductor for supplying plate energy to said grid, the neutralizing condenser being adjacent a plate and the neutralizing conductor being relatively long, and an inductor of substantially no resistance in parallel to said conductor connected adjacent the junction of said conductor and said condenser.

9. In a push-pull, ultra high frequency power amplifier, including a pair of thermionic tubes, a pair of neutralizing condensers adjacent the plates of said tubes and relativel long neutralizing conductors connecting the neutralizing condenser of one tube to the control grid of the other tube, and an inductor of substantially no resistance connecting said conductors adjacent their junction with their associated condensers.

10. In a push-pull ultra high frequency power amplifier including a pair of thermionic tubes, at least one neutralizing condenser and one neutraL izing conductor connecting the plate of each tube tothe;controlgridpf 'th etotliertube,-1thew combinaamplifier including ,a pair of thermionic tubes, a 'pair of neutralizing condensers and neutralizing conductors connecting theplate of teachitube to the control grid of the other tube, the combination with said neutralizing elements of one inductor'arranged in-parallel with the distributive capacity of one :of said conductors and one of said .condensersand connected adjacent the junction ofv said one conductor and said one condenser.

12. In. an amplifier-including a thermionic tube operable at-a desired frequency and a neutralizing condenser and a neutralizing conductor for supplying plate energy to a control grid, the combination with said neutralizing elements of an inductor ,arrangedin parallel with the vdistributive capacity of said conductorandcondensensaid inductor and distributive capacity comprising elements of a-parallel tuned circuit tuned to said desired frequency,

13. In a push-pull amplifier including a pairof thermionic tubes operable at a desired frequency, at least one neutralizing condenser'and ,one neutralizing conductor connecting the plate of each tube to the control grid of the other tube, the combination With said neutralizing elements of an inductor connected in parallel With the distributive capacity of said conductor and condenser, said inductor and distributive capacity comprising elements of a parallel tuned circuit tuned to said desired frequency.

WILLIS E. PHILLIPS. 

